2020 GMC Sierra 1500 Transmission & Axle Gear Ratios

Transmission gear ratios

The 2020 GMC Sierra 1500 was offered with a variety of transmissions, each designed to optimize performance and fuel economy for different applications. The available transmissions included an 8-speed automatic and a 10-speed automatic.

8-speed Automatic Transmission (MQE)

10-speed Automatic Transmission (MQB)

Available axle ratios

The 2020 GMC Sierra 1500 offered several axle ratios, allowing buyers to tailor their truck for specific needs, whether it's maximizing towing capability or optimizing fuel efficiency.

How to identify your axle ratio

For GMC vehicles like the Sierra 1500, the easiest way to identify your axle ratio is by checking the Service Parts Identification (RPO) sticker. This sticker is typically found on the inside of the glove box door or sometimes on the driver's side door jamb. Look for three-character codes that start with "GU".

• GU4: Indicates a 3.23 rear axle ratio.
• GU6: Indicates a 3.42 rear axle ratio.
• GT4: Indicates a 3.73 rear axle ratio.

If the RPO sticker is missing or unreadable, a driveline shop can often determine the ratio by inspecting the axle tag (if present on the differential housing) or by performing a differential rotation count.

Towing vs MPG impact

The choice of axle ratio significantly impacts both towing performance and fuel economy. Generally, a numerically lower axle ratio (e.g., 3.23) means the engine will operate at lower RPMs at a given road speed, resulting in better fuel economy. However, it will also reduce the truck's ability to accelerate with heavy loads and decrease its maximum towing capacity.

Conversely, a numerically higher axle ratio (e.g., 3.73) means the engine will turn at higher RPMs, providing more torque to the wheels. This enhances towing capacity and acceleration, making it ideal for hauling heavy loads or frequent towing. The trade-off is typically a reduction in fuel efficiency.

The 3.42 ratio offers a good balance between these two extremes, providing decent towing capability without a severe penalty in fuel economy, which is why it's a popular choice for many Sierra 1500 owners.

Common issues

While the transmissions and axles in the 2020 GMC Sierra 1500 are generally robust, a few common issues can arise over time:

• Ring and Pinion Whine: A persistent whining noise that changes with vehicle speed (and often pitch when accelerating or decelerating) can indicate wear or improper setup of the differential's ring and pinion gears. This often requires professional diagnosis and potentially a rebuild or replacement of the differential.
• Carrier Bearing Wear: The bearings supporting the differential carrier can wear out, leading to a humming or growling noise, especially at certain speeds. If left unaddressed, it can lead to more significant damage within the differential.
• Limited-Slip Clutch Wear: For vehicles equipped with a limited-slip differential, the internal clutches can wear down over time. Symptoms might include a clunking noise during tight turns or a noticeable loss of traction in low-traction situations where the limited-slip feature should engage.

Find a driveline shop on Repairius.